The present invention relates to an improvement in a rubber or plastic insulated power cable.
A power cable insulated with rubber or plastics to withstand high voltages of 3,300 V or more has a basic structure as shown in FIG. 1. Referring to FIG. 1, a conductor shield 2, an insulation layer 3 of polyethylene, crosslinked polyethylene, ethylene-propylene rubber, butyl rubber or the like, an insulation shield 4, a metal shield layer 5 of a copper wire, a copper tape or the like, and, if necessary, a jacket 6 of a polyvinyl chloride composition or the like are formed in turn in the order named around a conductor 1.
In a power cable of the structure described above, water infiltration may occur either along the conductor or from the outside to the inside of the cable, through the terminal, connecting portions, and/or the outer jacket of the cable for any of a number of reasons arising during manufacture, storage, installation or use of the cable. Water may penetrate from the conductor to the conductor shield and thence to the insulation layer. When AC voltage is applied to a cable installation into which water has been infiltrated in this manner, fine defects called water-trees are formed in the insulation layer and the semiconductive shield. This degrades insulation performance of the cable and may cause an electrical failure.
Methods for preventing water infiltration into the cable insulation or the like may be roughly divided into category (A) methods for preventing water from infiltrating radially into the cable, and category (B) methods for preventing water infiltration along the conductor.
Category (A) methods include (1) a method in which a metal/plastic laminated tape is placed beneath a sheath with the plastic side facing the sheath and the jacket and the plastic layer are bonded together during jacket extrusion so as to form a water impervious layer, and (2) a method in which a watertight compound is introduced beneath the sheath. However, in practice, a satisfactory waterproof effect can not be obtained by these methods.
More specifically, with the method (1), when the jacket has a defect such as a crack, a hole or the like due to either mechanical impact or mishandling during installation, the water impervious layer which is integral with the jacket is also damaged. Then, water enters the cable through the damaged portion and penetrates into the insulation shield or the insulation layer, resulting in unsatisfactory water proofness. With the method (2), the storage life of the watertight compound over a long period of time has not been confirmed. In addition, filling the the gaps between the jacket and the core completely is difficult. For this reason, water infiltration along the longitudinal direction of the cable cannot be satisfactorily prevented. Quality of the power cable cannot be guaranteed.
Category (B) methods include the introduction of a homogeneous mixture of a low-molecular weight polyethylene, microcrystalline wax, polybutene petrolatum, or the like, or a homogeneous mixture of polyvinyl chloride, natural rubber, butyl rubber or the like with a softening agent, as infilling between the stranded wires so as to obtain a watertight construction of stranded conductor. However, a watertight homogeneous compound for filling conductors is generally strongly thixotropic. For this reason, when the wires are stranded, the homogeneous mixture must be troweled while being heated at a high temperature or must be injected under high pressure. Therefore, the mixture becomes scattered around the stranding machine, significantly polluting the working environment. With such a stranding step including the introduction of such a compound, stranding speed is significantly decreased. In addition to these difficulties encountered during manufacture, the filled compound must be completely removed when two cables are to be connected. Connection of cables is thus rendered difficult.